1. Field of the Invention
The present invention relates to exposure apparatuses and an exposure method, and more particularly, to exposure apparatuses and an exposure method, by which a photosensitive material is exposed with a plurality of exposure heads.
2. Description of the Related Art
Recently, there have been various kinds of exposure systems with exposure heads in which light emitting elements are arranged in a one- or two-dimensional manner. In these exposure systems, a wider exposure head is required in order to expose a larger area. However, it is obvious that a larger size of an exposure head requires a remarkable increase in its cost. For example, it may be assumed that the cost is increased not twice, but three to five times when the exposure area is doubled,
One method for avoiding a lager size of the exposure head has been considered to expose a larger area with a multihead system in which a plurality of exposure heads are joined in the main scanning direction. However, there has been caused a problem that quality in an image is deteriorated, because simple uniting of the exposure heads causes “stripe-shaped unevenness” in a part corresponding to a joining position of exposed images by a wider pitch between the light emitting elements in the joining part as shown in FIG. 9 or, by a deviation in relative positions between the exposure heads, and then the image with a joint is obtained.
In order to solve such a problem, there has been proposed a multihead writing device (Refer to the Japanese Patent Application Laid-Open No. 2000-15864) in which a plurality of exposure heads are arranged in a staggered manner, and the distance between the light emitting elements in each end part of exposure heads adjacent to each other is made approximately equal to the pitch for the other light emitting elements.
However, “stripe-shaped unevenness” is caused by unevenness in the light-emitting intensity among the exposure heads even if a plurality of the exposure heads are arranged with high accuracy in position adjustment, and then an exposed image with a joint is obtained. Moreover, it is difficult to remove the unevenness in the light-emitting intensity among the exposure heads. For example, even if the light-emitting intensity of the exposure heads is made equal between them at manufacturing the exposure apparatuses, long use of the heads causes the unevenness in the light-emitting intensity among the heads. Moreover, since the light emitting intensity is changed by an environmental temperature at which the exposure apparatuses is used, the unevenness in the light emitting intensity among the exposure heads is caused.